There are known two general methods for accomplishing bending work on metal pipes, rods, or the like. In cold bending the desired bending work is performed by giving to the work piece a bending moment that exceeds the elastic limit of the work piece at normal temperature. In hot bending the bending work is accomplished by giving a bending moment to the work piece by heating it to a temperature that induces plastic deformation of the work piece. Cold bending is suited for relatively small diameter metal pipes or the like of which no high product precision is required, but it is not suited for relatively large diameter metal pipes or the like of which high product precision is required. On the other hand, hot bending is suited for relatively large diameter metal pipes or the like of which high product precision is required, but fairly large-scale equipment and heating apparatus is required. Also, as work efficiency is poor, the working cost is high.
Attempts have been made to overcome these problems, and there have been recently developed and put to practical use an improved method and apparatus for effecting bending work by hot bending, according to which a metal pipe or the like to be worked is first passed through a heating device such as for example a high frequency inductor which is capable of effecting high temperature heating over a limited area, with the end or a suitable middle part of said pipe being clamped to an arm which is freely swingable and whose axis of revolution is located within the plane of said heating device, the arm also having a length that matches the bending radius of said pipe. Then, with the pipe being continuously driven straight forward, it is subjected to local heating to a plastic deformation inducing temperature by said heating device, which is immediately followed by cooling so as to effect continuous plastic deformation in said heated area of said pipe while giving a bending moment to said pipe, thereby to accomplish the desired bending.
According to this method and apparatus, the desired bending can be accomplished on solid metal materials very efficiently with high precision, but when hollow metal pipes are subjected to bending, there takes place in some cases a phenomenon detrimental to the product, such as flattening, flexing or buckling in the pipe, and this may cause a change of curvature increasing or reducing the bending radius of the pipe (such phenomenon being hereinafter referred to as "R flow"). Also, when the clamp is released upon completion of the bending, there may occur so-called spring back to cause expansion of the bending radius (hereinafter referred to as "R expansion").
In recent years, pipelines are popularly used for the transportation of fluids, and the number of steel pipes used for such pipelines is increasing at a high rate. Also, more and more high precision is required for bending such pipes.
Thus, demand has been voiced in the industries for development of a method and apparatus which are capable of effecting bending metal pipes such as steel pipes with higher precision than is attainable with the presently available techniques.
In view of the above, the present invention has for its object to provide a method and apparatus which are capable of bending long metal materials such as pipe, bar and rod with high efficiency and high precision.
It is another object of the present invention to provide a method and apparatus which are capable of bending steel pipes used for constructing pipelines for fluid transport without causing any undesirable phenomenon such as flattening, flexing or buckling, that is, R flow or R expansion.